The heart is divided into four chambers, the left and right atrial chambers and the left and right ventricular chambers. As the heart beats, the atrial chambers and the ventricular chambers go through a cardiac cycle. The cardiac cycle consists of one complete sequence of contraction and relaxation of the chambers of the heart. The terms systole and diastole are used to describe the contraction and relaxation phases the chambers of the heart experience during a cardiac cycle. In systole, the ventricular muscle cells contract to pump blood through the circulatory system. During diastole, the ventricular muscle cells relax, causing blood from the atrial chamber to fill the ventricular chamber. After the period of diastolic filling, the systolic phase of a new cardiac cycle is initiated.
Through the cardiac cycle, the heart pumps blood through the circulatory system. Effective pumping of the heart depends upon five basic requirements. First, the contractions of cardiac muscle must occur at regular intervals and be synchronized. Second, the valves separating the chambers of the heart must fully open as blood passes through the chambers. Third, the valves must not leak. Fourth, the contraction of the cardiac muscle must be forceful. Fifth, the ventricles must fill adequately during diastole.
When the contractions of the heart are not occurring at regular intervals or are unsynchronized the heart is said to be arrhythmic. During an arrhythmia, the heart's ability to effectively and efficiently pump blood is compromised. Many different types of arrhythmias have been identified. Arrhythmias can occur in either the atrial chambers or in the ventricular chambers of the heart.
Ventricular tachycardia is an arrhythmia that occurs in the ventricular chambers of the heart. Ventricular tachycardias are typified by ventricular rates between 120–250 and are caused by disturbances (electrical or mechanical) within the ventricles of the heart. During a ventricular tachycardia, the diastolic filling time is reduced and the ventricular contractions are less synchronized and therefore less effective than normal. Ventricular tachycardias must be treated quickly in order to prevent the tachycardia from degrading into a life threatening ventricular fibrillation.
Arrhythmias that occur in the atrial chambers of the heart are referred to generally as supraventricular tachycardias. Supraventricular tachycardias include atrial tachycardias, atrial flutter and atrial fibrillation. During certain supraventricular tachycardias, aberrant cardiac signals from the atria drive the ventricles at a very rapid rate. Such a situation occurs during paroxysmal atrial tachycardia. This condition begins abruptly, lasts for a few minutes to a few hours, and then, just as abruptly, disappears and the heart rate reverts back to normal.
Cardioverter-defibrillators, such as implantable cardioverter-defibrillators (ICDs), have been shown to be effective in reducing the incidence of sudden cardiac death. Sudden cardiac death is typically caused by either ventricular tachycardia or ventricular fibrillation. Cardioverter-defibrillator systems operate by sensing and analyzing cardiac signals and applying electrical energy to the heart when either a ventricular tachycardia or ventricular fibrillation is detected.
One common way cardioverter-defibrillators detect cardiac arrhythmias is to sense and analyze the rate of ventricular contractions. When the ventricular rate exceeds a programmed threshold value, the cardioverter-defibrillator applies electrical energy in one or more specific patterns to treat either the ventricular tachycardia or ventricular fibrillation.
An additional method cardioverter-defibrillators use to detect cardiac arrhythmias is to compare the morphology of sensed cardiac complexes to template cardiac complexes representative of specific cardiac rhythms. As each cardiac complex is sensed, it is compared to the template cardiac complexes in an effort to identify and classify the sensed cardiac complex. Template cardiac complexes can be representative of a variety of cardiac complexes, including ventricular tachycardias and normal sinus rhythm.
Template cardiac complexes are typically programmed into an implantable medical device shortly before or after the device has been implanted into the patient. Once the implantable medical device has been implanted into the patient, however, the physiologic environment in which cardiac electrodes are placed (i.e., the heart) begins to change. These changes can include an inflammatory response, localized fibrosis around the implanted electrode and cardiac disease progression. These physiological changes lead to a deterioration, or a change in the strength and the morphology of the signal sensed by the implanted medical device. Additionally, changes in a patient's medication regimen can also change the sensing of cardiac signals by the implanted medical device. Therefore, cardiac complex templates developed before or soon after implanting the medical device can become less useful, or reliable, in the process of assessing and classifying unknown cardiac complexes.
Therefore, a need exists for addressing the changes in sensed cardiac signals as the physiological environment surrounding implanted cardiac electrodes changes.